session_timeout not working

[vinayp]

Our customer requires an installation of roundcube that times out on inactivity of 20 minutes. I have tried session_timeout to even values as low as 5 but it is inconsistent behaviour. It times out sometimes but stops working suddenly after few days. This makes me believe that some other setup is affecting it. I have even tried keeping the message refresh setting in Settings->User interface to never. But it does not help.
Any suggestions on how to make session_timeout work for inactivity?